The degree of histocompatibility existing between donor and host has been strongly implicated as a major determinant of the effectiveness of hematopoietic and immunologic reconstitution. While the tempo and quality of reconstitution following transplantation of marrow from HLA- matched related donors, and the infectious complications inherent in its failure of re-develop have been well documented, there is a yet insufficient information regarding immunologic reconstitution in recipients of marrow derived from unrelated or partially HLA-matched related donors exhibiting unique minor alloantigens or microvariant disparities within HLA. The cellular, genetic, and immunologic characteristics of the marrow graft and transplant recipient which incite or inhibit the cellular interactions leading to rejection or engraftment, leukemic relapse or resistance, and effective immune reconstitution or persistent immunodeficiency constitute the subjects of investigation proposed in the research projects and define the rationale for the program of experimental therapeutics. This core program for histocompatibility testing and immunologic monitoring is designed to provide 1) molecular definition of recognized HLA Class II variants, using established techniques incorporating PCR-SSOP, in order to complement the standard HLA A, B, and DR serologies currently used to identify histocompatible related and unrelated donors and 2) a centralized program for monitoring the course of immunologic reconstitution following allogenic marrow transplant. This will include characterizations of regenerating lymphoid populations distinguishable by specific immunologic phenotype, assessments of lymphoid functions including cytolytic activity, proliferative responses to mitogens, superantigens, and/or specific antigens, and the capacity to generate and respond to cytokines. The studies proposed within this core will provide a more detailed analysis of HLA Class II disparities existing between donor and host, and will provide baseline analysis for gauging the effect of degree of histoincompatability, alternative methods of cytoreduction, newer T-cell depletion techniques, graft rejection prophylaxis, and lymphohematopoietic cytokines on the redeveloping immune system.